1. Field of the Invention
The present invention relates to an optical receiver for receiving an optical signal modulated by M-ary (M≧4) DPSK, and specifically relates to an optical DQPSK receiver apparatus for receiving an optical signal modulated by DQPSK. The present invention is applicable to optical communications, optical signal processing, and optical measurements.
2. Description of the Related Art
Phase modulation has been in practical use as one of technologies to transmit signals in optical transmission systems. In the phase modulation, data is transmitted by shifting a phase of carrier wave in accordance with transmission data. For example, in Quadrature Phase Shift Keying (QPSK), respective “θ”, “θ+π/2”, “θ+π”, or “θ+3π/2” is assigned to each of the corresponding symbol “00”, “01”, “11” or “10” comprised by 2-bit data. Here, “θ” is an arbitrary phase. The receiver equipment recovers transmitted data by detecting the phase of the received signal.
Differential Quadrature Phase Shift Keying (DQPSK) is known as a technology to improve receiver sensitivity of a QPSK receiver. In DQPSK, the amount of change in the phase of a carrier wave between two consecutive symbols (“0”, “π/2”, “π”, or “3π/2”) is associated with the 2-bit transmission information. Therefore, the receiver equipment can recover transmission data by detecting the phase difference between the adjacent two symbols. By the DQPSK modulation, increase in transmission speed and/or improvement of optical S/N ratio, which is deteriorated due to the increase in speed in the transmission path, can be achieved.
FIG. 1 is a diagram describing the configuration of an existing optical DQPSK receiver apparatus. An optical DQPSK receiver apparatus 1000 shown in FIG. 1 comprises a pair of interferometers 1001 and 1002, and an optical DQPSK signal is split by an optical splitter and guided to a pair of the interferometers 1001 and 1002. A one-symbol delay element is provided to one arm of the interferometer 1001, and a π/4 phase shift element is provided to the other arm. A one-symbol delay element is provided to one arm of the interferometer 1002, and −π/4 phase shift element is provided to the other arm. A photo detector circuit 1003 comprises a pair of photodiodes 1003a and 1003b connected in series with each other. A photo detector circuit 1004 comprises a pair of photodiodes 1004a and 1004b connected in series with each other. A pair of optical signals output from the interferometer 1001 is guided to the photodiodes 1003a and 1003b, and a pair of optical signals output from the interferometer 1002 is guided to the photodiodes 1004a and 1004b. Note that “to connect in series” refers to the state of a pair of two photodiodes connected in series when the two diodes are modeled as current sources.
Each of the photo detector circuits 1003 and 1004 outputs a difference in current generated by each of a pair of photodiodes (differential signals). The differential signals generated by the photo detector circuits 1003 and 1004 are amplified by each of preamplifiers 1005 and 1006, and are transmitted to a signal process circuit, not shown in the drawing. The signal process circuit recovers 2-bit data from these differential signals.
FIG. 2 is a diagram explaining operations of the optical DQPSK receiver apparatus shown in FIG. 1. In DQPSK, the phase difference between adjacent symbols is “0”, “π/2”, “π”, or “3π/2”. Thus, the phase difference between a pair of optical signals interfering each other in the interferometer 1001 is “π/4”, “3π/4”, “5π/4”, or “7π/4”. The phase difference between a pair of optical signals interfering each other in the interferometer 1002 is “7π/4”, “π/4”, “3π/4”, or “5π/4”. If the phase differences in the optical signals interfering with each other in the interferometers 1001 and 1002 are defined “Δφa” and “Δφb”, respectively, the current generated by each photodiode is as shown in FIG. 2. In other words, if the phase difference between adjacent symbols is “0”, for example, the current generated by the photodiode 1003a is “0.15”. Note that the current value shown in FIG. 2 is normalized.
Each of the photo detector circuits 1003 and 1004 outputs signal representing the difference between the current generated by a pair of photodiodes. As a result, the output levels of the photo detector circuits 1003 and 1004 becomes “0.7” or “−0.7” in accordance with the phase difference between symbols adjacent to each other. Therefore, by using “threshold=0”, 2-bit data corresponding to the phase difference between the symbols can be obtained. In the example of FIG. 2, for example, when the phase difference between symbols is “π/2”, the output levels of the photo detector circuits 1003 and 1004 are “−0.7(<0)” and “0.7(>0)”, and therefore, “0” and “1” are obtained.
The configuration and the operation of the optical DQPSK receiver apparatus are described in detail in Patent Document 1 (Published Japanese Translation of PCT patent application No. 2004-516743) and Patent Document 2 (WO 03/063515A2), for example.
In the conventional optical DQPSK receiver apparatus shown in FIG. 1, when the amount of phase shift of the phase shift element in an interferometer deviates, reception quality is degraded. For that reason, it is necessary to accurately adjust the amount of phase shift of the phase shift element all the time. However, because the amount of phase shift of the phase shift element changes due to the thermal change and aging degradation etc., the control of the amount of phase shift is complicated.
There is a requirement to reduce the size of the optical DQPSK receiver apparatus. In order to meet the requirement, it is necessary to reduce the number of the parts constituting the optical circuit (mainly interferometers and photo detector circuits).
In addition, when the optical DQPSK receiver apparatus receives high-speed data of several ten Gbps, the frequency characteristics of the photodiode constituting the photo detector circuits 1003 and 1004 have to be superior. However, such a high-speed photodiode is very expensive in general. In other words, if the number of photodiode constituting the photo detector circuit can be reduced, it is possible to keep the cost of the optical DQPSK receiver apparatus low.